This application claims benefit of Japanese Application(s) No. 2000-15271 filed in Japan on Jan. 25, 2000 and No. 2000-181014 filed in Japan on Jun. 16, 2000, the contents of which are incorporated by this reference.
The present invention relates generally to a zoom lens, and more particularly a zoom lens system designed for use on compact cameras.
Among compact camera-oriented zoom lenses well known in the art, there are a two-group zoom lens designed to vary its focal length by varying the separation between a positive group and a negative group and a three-group zoom lens designed to vary its focal length by varying separations between three groups, positive, positive and negative.
Of these zoom lenses, the two-group zoom lens can be more simplified in lens mount structure and driving mechanism, and more reduced in the number of lenses, than the three-group zoom lens, and so is favorable for achieving cost reductions. On the other hand, the three-group zoom lens has a high zoom ratio and is excellent in overall size reductions.
Compact zoom lens systems having relatively high zoom ratios are disclosed in JP-A""s 8-152559, 10-268191, 11-52243 and 11-119098.
With the zoom lenses set forth in these publications, it is possible to achieve some size reductions as can be seen from the telephoto ratio of about 0.08. However, all of them have a zoom ratio of 3 or greater and a focal length of 130 mm or greater on the telephoto side, and so still need a large camera lens mount even when the telephoto ratio is reduced down to 0.08. Thus, these zoom lenses fail to reduce the size of associated cameras. Usually, the higher the zoom ratio, the larger the size Of an associated camera is. For this reason, cameras having a zoom ratio of 3 or greater is vastly inferior in compactness to 2xc3x97 class cameras; that is, users who attach importance to compactness must wink at specifications to some degrees.
To achieve further size reductions by increasing zoom ratios, it is generally preferable to increase the curvature and, hence, refracting power of each lens surface. In this case, however, large aberrations are produced at each lens surface and aberration fluctuations during zooming become large. To obtain a sufficient zoom ratio and reduce the aberration fluctuations during zooming, thereby achieving satisfactory performance throughout zooming zones, it is required to increase the number of lenses and, optionally, make heavy use of aspherical surfaces. This results not only in an increase in the size of the lens system but also in cost increases.
When the aberration fluctuations during zooming are reduced by decreasing the refracting power of each lens, the amount of zooming movement of each group becomes too large to make the lens system compact.
In view of such problems with the prior art as mentioned above, an object of the present invention is to provide a very compact zoom lens system which has a high zoom ratio and improved performance.
According to the first aspect of the present invention, there is provided a zoom lens system comprising, in order from an object side of the system, a positive, first lens group, a positive, second lens group and a negative, third lens group, characterized in that while each lens group moves from an image side of the system to the object side for zooming from a wide-angle end to a telephoto end of the system, the following condition (1) is satisfied:
LT/fT less than 0.75xe2x80x83xe2x80x83(1)
Here LT is the distance at the telephoto end from the front surface of the zoom lens system to an image formation position and fT is the focal length of the zoom lens system at the telephoto end.
It is here noted that by the front surface of the zoom lens system is intended the object-side surface of the zoom lens system located nearest to the object side of the zoom lens system and by the image formation position is intended a position where an object image is formed by the zoom lens system and a film plane or an image pickup device such as a CCD is located.
According to the second aspect of the present invention, there is provided a zoom lens system comprising, in order from an object side of the system, a positive, first lens group, a negative, second lens group, a positive, third lens group and a negative, fourth lens group, characterized in that while each lens group moves from an image side of the system to the object side for zooming from a wide-angle end to a telephoto end of the system, the following condition (1xe2x80x2) is satisfied:
LT/fT less than 0.65xe2x80x83xe2x80x83(1xe2x80x2)
Here LT is the distance at the telephoto end from the front surface of the zoom lens system to an image formation position and fT is the focal length of the zoom lens system at the telephoto end.
According to the third aspect of the present invention, there is provided a zoom lens system comprising, in order from an object side of the system, a positive, first lens group, a positive, second lens group and a negative, third lens group, characterized in that while each lens group moves from an image side of the system to the object side for zooming from a wide-angle end to a telephoto end of the system, the first lens group comprises, in order from an object side thereof, a positive lens and a negative lens, and satisfies the following condition (2):
0.2 less than f1/fT less than 0.5xe2x80x83xe2x80x83(2)
Here f1 is the focal length df the first lens group and fT is the focal length of the zoom lens system at the telephoto end.
According to the fourth aspect of the present invention, there is provided a zoom lens system comprising, in order from an object side of the system, a positive, first lens group, a positive, second lens group and a negative, third lens group, characterized in that while each lens group moves from an image side of the system to the object side for zooming from a wide-angle end to a telephoto end of the system, the second lens group comprises, in order from an object side thereof, a cemented lens consisting of a negative lens and a positive lens and a positive lens and the following condition (3) is satisfied:
fT/fW greater than 3xe2x80x83xe2x80x83(3)
Here fT is the focal length of the zoom lens system at the telephoto end, and fW is the focal length of the zoom lens system at the wide-angle end.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.